1. Field of the Invention
The present invention relates to a high electron mobility transistor (HEMT). More particularly, the present invention relates to a HEMT formed as a field effect transistor (FET) having a gate insulating layer positioned between a gate contact and channel layer. The invention also pertains to manufacturing methods for the aforementioned HEMT device.
2. Description of the Related Art
As known to those familiar with semiconductors, materials such as silicon (Si) and gallium arsenide (GaAs) have found wide application in semiconductor devices for lower power and in the case of Si lower frequency applications. These more familiar semiconductor materials have failed to penetrate higher power high frequency applications to the extent desirable, however, because of their relatively small bandgaps (e.g., 1.12 eV for Si and 1.42 eV for GaAs at room temperature) and relatively small breakdown voltages.
Accordingly, interest in high power, high temperature, and high frequency applications and devices has turned to wide bandgap semiconductor materials such as zinc oxide (3.37 eV at room temperature), silicon carbide (SiC) (2.996 eV for alpha SiC at room temperature) and the Group III nitrides such as gallium nitride (GaN) (3.36 eV for GaN at room temperature). These materials have higher electric field breakdown strengths and higher electron saturation velocities as compared to GaAs and Si.
A device of particular interest is the high electron mobility transistor (HEMT), which is also known as a modulation-doped field effect transistor (MODFET). These devices offer operational advantages under a number of circumstances because a two-dimensional electron gas (2DEG) is formed at the heterojunction of two semiconductor materials with different bandgap energies, and where the smaller bandgap material has a higher electron affinity than the larger bandgap material. The 2DEG is an accumulation layer in the undoped, smaller bandgap material and can contain a very high sheet electron concentration on the order of 1012 to 1013 carriers per square centimeter (carriers/cm2). Additionally, electrons that originate in the doped, wider-bandgap semiconductor transfer to the 2DEG, allowing a high electron mobility due to reduced ionized impurity scattering.
This combination of high carrier concentration and high carrier mobility gives the HEMT a very large transconductance and a strong performance advantage over metal-semiconductor field effect transistors (MESFETs) for high-frequency applications. High electron mobility transistors fabricated in the zinc oxide/aluminum gallium nitride (ZnO/AlGaN) material system have the potential to generate large amounts of radio-frequency (RF) power because of their unique combination of material characteristics which includes the aforementioned high breakdown fields, their wide bandgaps, large conduction band offset, and high saturated electron drift velocity. A major portion of the electrons in the 2DEG is attributed to pseudomorphic strain in the AlGaN/MgZnO since the MgZnO in-plane lattice parameter is 3% smaller than that of ZnO. When (0001) oriented MgZnO alloys are grown on thick ZnO, the biaxial strain induces a piezoelectric field in the material. When MgZnO layers are grown beyond the critical thickness for the Mg concentration used, the film begins to relax, and if the strain is sufficient, it leads to cracking of the film. This strain is useful however, in that it can be used to create a piezoelectric field in the structure. Bykhovski et. al. proposed using piezoelectric doping for AlGaN/GaN (possible replacement of GaN by ZnO is suggested in the same work) HEMT structures as a substitute for conventional impurity doping. The piezoelectric doping produces a 2-D electron gas (2DEG) near the interface without having to use conventional doping. In order to work, a high quality MgZnO layer is required, because the electron density in the MgZnO/ZnO 2DEG structure is limited by the elastic strain relaxation, which depends on MgZnO barrier thickness and on the Mg molar fraction in the barrier. It is this principle that has allowed the development of ZnO based FETs. Strain diminishes once the barrier thickness is larger than the critical thickness. The development of misfit dislocations in heterostructures can significantly affect the mobility and reduce the device performance. Maeda et al. further discovered in their study that the maximum 2DEG density depends more strongly on the strain relaxation than on the Al composition.
High power semiconducting devices of this type operate in a microwave frequency range and are used for RF communication networks and radar applications and offer the potential to greatly reduce the complexity and thus the cost of cellular phone base station transmitters. Other potential applications for high power microwave semiconductor devices include replacing the relatively costly tubes and transformers in conventional microwave ovens, increasing the lifetime of satellite transmitters, and improving the efficiency of personal communication system base station transmitters. Accordingly, the need exists for continued improvement in high frequency high power semiconductor based microwave devices.